Sensors for detecting sub-ppm NO2 using photochemically produced amorphous tungsten oxide

This paper describes a new approach to making the active elements of gas sensors using a photochemical deposition method. The method involves making amorphous films of photochemically active precursors. These precursors are then exposed to light and, in air, convert to metal oxides. Amorphous films constructed of W(CO)(4)(Et-2-en) are deposited on interdigitated microelectrodes. Photolysis of these films, in air at room temperature, results in the deposition of amorphous films of tungsten oxide. This forms the sensing element of an NOx sensor. Films thus prepared were also annealed to yield sensors whose active elements were crystalline tungsten oxide. An investigation of the conduction mechanism in the sensor materials was performed. The conduction in the polycrystalline materials is controlled by grain boundaries through thermionic emission. Absorption of NO2 leads to a modification of the grain boundary resulting in the change in current. In contrast, the amorphous materials conduct via variable range hopping. In this case, absorption of NO2 leads to a reduction in the number of carriers and a change in conductivity of the material. The response of the amorphous materials has been fit to a model based on a Langmuir isotherm.